2.3 Transmission Lines – an Introduction
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2.3 Transmission Lines – an Introduction
We will start our treatment of transmission lines with one the most common types, the
coaxial cable. A coaxial cable is a two-conductor cable made of a single conductor surrounded by a
braided wire jacket, with a plastic insulating material separating the two. The outer conductor
completely surrounds the inner conductor and the two conductors are insulated from each other for
the entire length of the cable.
Figure 2.3-1 Coaxial cable construction
If an ohmmeter was employed to check the cable's resistance, it would show the two
conductors to be completely insulated from one other, with nearly infinite resistance between the
two. This is due to the fact that an ohmmeter would use a continuous direct current (DC) to perform
such a measurement. However, the cable's response to short voltage pulses and high-frequency
signals would be quite different because of the effects of capacitance and inductance distributed
along the length of the cable. When the applied voltage changes rapidly, the cable presents a finite
impedance to the signal source (typically 50 or 75 Ω), and draws a current proportional to the
applied voltage. When such stimuli are used, this pair of wires becomes an important circuit element
with its own characteristic properties which we refer to as a transmission line.
Let us now consider a set of parallel wires of infinite length (Figure 2.3-2), with nothing
connected at the end. What would happen when we close the switch? Would there be no current at
all?
Figure 2.3-2 Driving an Infinite transmission line
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